It is often necessary to employ hybrid circuits to obtain desirable circuit performance at microwave frequencies. However, it is usually difficult and expensive to integrate hybrid circuits into a printed circuit board design. Another disadvantage of most designs which integrate hybrid circuits with printed circuit boards is that the hybrid circuit requires a lot of area on the printed circuit board.
A traditional approach to integrating hybrids and printed circuit boards is to package the hybrid circuit in a machined metal package. The metal packaged hybrid includes coaxial radio frequency (RF) connections as an interface to the printed circuit board. Both the metal package and the coaxial connections of this design approach are typically bulky and expensive.
Another approach that has been used is to package the hybrid in a multi-layer co-fired ceramic package. This approach is typically used to package an integrated circuit (IC) chip and a few other circuit components. These ceramic packages are offered in standard layout configurations which limits the design circuit elements and, hence, the design flexibility. This approach also tends to be expensive.
Yet another approach is to employ edge clips to connect a hybrid circuit to a printed circuit board. The disadvantage to this interface is that the electrical performance is poor, providing adequate operation up to only 1 or 2 GHz.
In accordance with the present invention, a surface-mount packageless hybrid (SMPH) is provided for direct attachment to a printed circuit board without additional carriers or packages. The SMPH includes an alumina thickfilm substrate. Circuit elements are attached or formed on the substrate using conventional processes and interconnected with microstrip. The entire back surface of the substrate is attached to a ground pad on the printed circuit board with solder or with conductive epoxy. Connections between the printed circuit board and the substrate are done in sets of three gull-wing shaped leads which connect three pads on the printed circuit board to three pads on the substrate. The pads provide capacitive matching sections to compensate for lead inductance. For an RF connection, the central lead of the three is a signal lead, and the outer two leads are ground leads. The same lead pattern is used for DC connections with each of the three leads capable of carrying a separate DC signal. The circuit elements on the substrate are protected with a cover or by encapsulation. However, the substrate is not enclosed in a package.
The SMPH provides an efficient method of integrating a hybrid circuit and a printed circuit board with minimal printed circuit board area requirements while maintaining a high performance RF transition and placing minimal restrictions on hybrid circuit design. With a packageless design, the SMPH is made less bulky and consumes less area on the printed circuit board. The SMPH provides transition compensation by means of the lead and pad design and by soldering or conductive epoxying the back of the substrate to the ground pad on the printed circuit board. Gross leak hermeticity is also provided. The surface mount packageless hybrid design is capable of high performance transitions in the lower microwave frequency region up to around 4 GHz. Higher frequency operation is also possible with less compensation.
SMPH hybrids have the further advantage of using standard hybrid circuit elements and assembly processes. SMPH hybrids are also compatible with standard surface mount printed circuit assembly processes.
Additional features and advantages of the invention will be made apparent from the following description of the preferred embodiment, which proceeds with reference to the accompanying drawings.